256 S. I. EMMONS — OROGRAPHIC MOVEMENTS. 



these two areas, owing to their almost unbroken covering of alluvial and 

 eruptive material ; but, as will be seen later, it may be inferred from the 

 structural conditions of the adjoining regions on the north and cast that 

 another elevated island once occupied some portion of it. possibly con- 

 nected with tin- southern end of the Sawatch island, which has disappeared 

 under the influence of erosion or local subsidence. 



A western meridional line of elevation beyond those above mentioned is 

 formed by the San Juan mountain- west of the San Luis park, the Elk 

 mountains west of the Sawatch range, and the White River plateau. The 

 two latt< r arc closely connected together, but arc separated from the greater 

 uplift of the San Juan mountains by the broad east and west depression of 

 the Gunnison valley. This line of elevation, as compared with that to the 

 east, is characterized by having been the scene of intense eruptive activity 

 in late Afesozoic and Tertiary times: and the same evidence of eruptive 

 activity is seen on the same north and south line in the Elkhead mountains 

 on the westein thinks of the Park range. 



It is only of the beds deposited during and subsequent to Cambrian times 

 that the outcrops are exposed in sufficient continuity to justify an attempt 

 at differentiating the land areas around which they were deposited. 



Pre-Cambria n Land. 



Of the extensive series of clastic sediments which the investigations of 

 [rving and his colleagues in the Lake Superior region have shown must 

 bave been deposited upon the Archaean basement of distinctly crystalline 

 rock- previous to the earliest Cambrian, for which the general term Algon- 

 kian is now proposed, only a few isolated exposures have yet been discovered 

 in the Rocky Mountain region, and these have not been sufficiently studied 

 to attempt any correlation between them. With regard to the earlier land 

 areas, therefore, only a few general conjectures can be formed. 



Algonkian Exposures. — Between the western Archaean continent (of which, 

 a- King has shown, the present Wasatch uplift must represent the eastern 

 Bhore-line and the Archaean islands of the Rocky Mountain region, it may 

 1m- assumed that a general depression existed in Algonkian time commen- 

 surate with that which has obtained in later periods. The Grand Cafion 

 and Chuar -'-nes, which Walcott has assumed to he of Algonkian age, and 

 on the upturned and eroded edges of which rest upper Cambrian beds, arc 

 on the general north and south line of the Wasatch uplift. Idie only other 

 known pre-Cambrian exposure in this depression i- that of the Red Creek 

 quartzites of the eastern Uinta mountains, which were classed a- Suronian 

 by the Fortieth Parallel geologists, and probably belong to one of the Algon- 

 kian Beries. They Berve to show that the Uinta uplift, which is ■>)' post- 



